XQuery

Chapter 3. Expressions: XQuery Building Blocks

The basic unit of evaluation in the XQuery language is the expression. A query contains expressions that can be made up of a number of sub-expressions, which may themselves be composed from other sub-expressions. This chapter explains the XQuery syntax, and covers the most basic types of expressions that can be used in queries: literals, variables, function calls, and comments.

Categories of Expressions

A query can range in complexity from a single expression such as 2+3, to a complex composite expression like a FLWOR. Within a FLWOR, there may be other expressions, such as $prodDept = "ACC", which is a comparison expression, and doc("catalog.xml")/catalog/product, which is a path expression. Within these expressions, there are further expressions, such as "ACC", which is a literal, and $prodDept, which is a variable reference.

The categories of expressions available are summarized in Table 3-1, along with the number of the chapter that covers them. Every expression evaluates to a sequence, which may be a single atomic value, a single node, the empty sequence, or multiple atomic values and/or nodes.

Table 3-1. Categories of expressions

Category	Description	Operators or keywords	Chapter
Primary	The basics: literals, variables, function calls, and parenthesized expressions		3
Comparison	Comparison based on value, node identity, or document order	=, !=, <, <=, >, >=, eq, ne, lt, le, gt, ge, is, <<, >>	3
Conditional